Microporous polishing pads

ABSTRACT

The invention provides polishing pads for chemical-mechanical polishing comprising a porous foam and a method for their production. In one embodiment, the porous foam has an average pore size of about 50 μm or less, wherein about 75% or more of the pores have a pore size within about 20 μm or less of the average pore size. In another embodiment, porous foam has an average pore size of about 20 μm or less. In yet another embodiment, the porous foam has a multi-modal pore size distribution. The method of production comprises (a) combining a polymer resin with a supercritical gas to produce a single-phase solution and (b) forming a polishing pad from the single-phase solution, wherein the supercritical gas is generated by subjecting a gas to an elevated temperature and pressure.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This patent application is a divisional of copending U.S. patentapplication Ser. No. 10/281,782, filed Oct. 28, 2002. This patentapplication claims the benefit of U.S. Provisional Patent ApplicationNo. 60/382,739, filed May 23, 2002.

FIELD OF THE INVENTION

[0002] This invention pertains to polishing pads for chemical-mechanicalpolishing comprising a porous foam having a uniform pore sizedistribution.

BACKGROUND OF THE INVENTION

[0003] Chemical-mechanical polishing (“CMP”) processes are used in themanufacturing of microelectronic devices to form flat surfaces onsemiconductor wafers, field emission displays, and many othermicroelectronic substrates. For example, the manufacture ofsemiconductor devices generally involves the formation of variousprocess layers, selective removal or patterning of portions of thoselayers, and deposition of yet additional process layers above thesurface of a semiconducting substrate to form a semiconductor wafer. Theprocess layers can include, by way of example, insulation layers, gateoxide layers, conductive layers, and layers of metal or glass, etc. Itis generally desirable in certain steps of the wafer process that theuppermost surface of the process layers be planar, i.e., flat, for thedeposition of subsequent layers. CMP is used to planarize process layerswherein a deposited material, such as a conductive or insulatingmaterial, is polished to planarize the wafer for subsequent processsteps.

[0004] In a typical CMP process, a wafer is mounted upside down on acarrier in a CMP tool. A force pushes the carrier and the wafer downwardtoward a polishing pad. The carrier and the wafer are rotated above therotating polishing pad on the CMP tool's polishing table. A polishingcomposition (also referred to as a polishing slurry) generally isintroduced between the rotating wafer and the rotating polishing padduring the polishing process. The polishing composition typicallycontains a chemical that interacts with or dissolves portions of theuppermost wafer layer(s) and an abrasive material that physicallyremoves portions of the layer(s). The wafer and the polishing pad can berotated in the same direction or in opposite directions, whichever isdesirable for the particular polishing process being carried out. Thecarrier also can oscillate across the polishing pad on the polishingtable.

[0005] Polishing pads used in chemical-mechanical polishing processesare manufactured using both soft and rigid pad materials, which includepolymer-impregnated fabrics, microporous films, cellular polymer foams,non-porous polymer sheets, and sintered thermoplastic particles. A padcontaining a polyurethane resin impregnated into a polyester non-wovenfabric is illustrative of a polymer-impregnated fabric polishing pad.Microporous polishing pads include microporous urethane films coatedonto a base material, which is often an impregnated fabric pad. Thesepolishing pads are closed cell, porous films. Cellular polymer foampolishing pads contain a closed cell structure that is randomly anduniformly distributed in all three dimensions. Non-porous polymer sheetpolishing pads include a polishing surface made from solid polymersheets, which have no intrinsic ability to transport slurry particles(see, for example, U.S. Pat. No. 5,489,233). These solid polishing padsare externally modified with large and/or small grooves that are cutinto the surface of the pad purportedly to provide channels for thepassage of slurry during chemical-mechanical polishing. Such anon-porous polymer polishing pad is disclosed in U.S. Pat. No.6,203,407, wherein the polishing surface of the polishing pad comprisesgrooves that are oriented in such a way that purportedly improvesselectivity in the chemical-mechanical polishing. Also in a similarfashion, U.S. Pat. Nos. 6,022,268, 6,217,434, and 6,287,185 disclosehydrophilic polishing pads with no intrinsic ability to absorb ortransport slurry particles. The polishing surface purportedly has arandom surface topography including microaspersities that have adimension of 10 μm or less and formed by solidifying the polishingsurface and macro defects (or macrotexture) that have a dimension of 25μm or greater and formed by cutting. Sintered polishing pads comprisinga porous open-celled structure can be prepared from thermoplasticpolymer resins. For example, U.S. Pat. Nos. 6,062,968 and 6,126,532disclose polishing pads with open-celled, microporous substrates,produced by sintering thermoplastic resins. The resulting polishing padspreferably have a void volume between 25 and 50% and a density of 0.7 to0.9 g/cm³. Similarly, U.S. Pat. Nos. 6,017,265, 6,106,754, and 6,231,434disclose polishing pads with uniform, continuously interconnected porestructures, produced by sintering thermoplastic polymers at highpressures in excess of 689.5 kPa (100 psi) in a mold having the desiredfinal pad dimensions.

[0006] In addition to groove patterns, polishing pads can have othersurface features to provide texture to the surface of the polishing pad.For example, U.S. Pat. No. 5,609,517 discloses a composite polishing padcomprising a support layer, nodes, and an upper layer, all withdifferent hardness. U.S. Pat. No. 5,944,583 discloses a compositepolishing pad having circumferential rings of alternatingcompressibility. U.S. Pat. No. 6,168,508 discloses a polishing padhaving a first polishing area with a first value of a physical property(e.g., hardness, specific gravity, compressibility, abrasiveness,height, etc.) and a second polishing area with a second value of thephysical property. U.S. Pat. No. 6,287,185 discloses a polishing padhaving a surface topography produced by a thermoforming process. Thesurface of the polishing pad is heated under pressure or stressresulting in the formation of surface features.

[0007] Polishing pads having a microporous foam structure are commonlyknown in the art. For example, U.S. Pat. No. 4,138,228 discloses apolishing article that is microporous and hydrophilic. U.S. Pat. No.4,239,567 discloses a flat microcellular polyurethane polishing pad forpolishing silicon wafers. U.S. Pat. No. 6,120,353 discloses a polishingmethod using a suede-like foam polyurethane polishing pad having acompressibility lower than 9% and a high pore density of 150 pores/cm²or higher. EP 1 108 500 A1 discloses a polishing pad of micro-rubberA-type hardness of at least 80 having closed cells of average diameterless than 1000 μm and a density of 0.4 to 1.1 g/ml.

[0008] Although several of the above-described polishing pads aresuitable for their intended purpose, a need remains for an improvedpolishing pad that provides effective planarization, particularly insubstrate polishing by chemical-mechanical polishing. In addition, thereis a need for polishing pads having improved polishing efficiency,improved slurry flow across and within the polishing pad, improvedresistance to corrosive etchants, and/or improved polishing uniformity.Finally, there is a need for polishing pads that can be produced usingrelatively low cost methods and which require little or no conditioningprior to use.

[0009] The invention provides such a polishing pad. These and otheradvantages of the invention, as well as additional inventive features,will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

[0010] The invention provides polishing pads for chemical-mechanicalpolishing comprising a porous foam. In a first embodiment, the porousfoam has an average pore size of about 50 μm or less, wherein about 75%or more of the pores have a pore size within about 20 μm or less of theaverage pore size. In a second embodiment, the polishing pad comprises aporous foam with an average pore size of about 1 82 m to about 20 μm. Ina third embodiment, the polishing pad comprises a thermoplasticpolyurethane foam having an average pore size of about 50 μm or less,wherein the thermoplastic polyurethane has a Melt Flow Index of about 20or less, a molecular weight of about 50,000 g/mol to about 300,000g/mol, and a polydispersity index of about 1.1 to about 6. In a fourthembodiment, the polishing pad is a polyurethane polishing pad comprisinga polishing surface with no externally produced surface texture, whichcan polish a silicon dioxide wafer at a rate of at least 600 Å/min witha carrier downforce pressure of about 0.028 MPa (4 psi), a slurry flowrate of about 100 ml/min, a platen rotation speed of about 60 rpm, and acarrier rotation speed of about 55 rpm to about 60 rpm. In a fifthembodiment, the polishing pad comprises a porous foam having amulti-modal pore size distribution, wherein the multi-modal distributionhas about 20 or less pore size maxima.

[0011] The invention further provides a method for producing thepolishing pads comprising (a) combining a polymer resin with asupercritical gas to produce a single-phase solution, wherein thesupercritical gas is generated by subjecting a gas to an elevatedtemperature and pressure, and (b) forming a polishing pad from thesingle-phase solution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a scanning electron microscopy (SEM) image (100×magnification) of a cross-section of an extruded porous foam rodproduced with a CO₂ concentration of 1.26% and a melt temperature of212° C. (414° F.).

[0013]FIG. 2 is a plot of carbon dioxide concentration versus densityillustrating the relationship between the concentration of CO₂ in asingle-phase solution of polymer resin and the density of the resultingporous foam prepared therefrom.

[0014]FIG. 3 is a scanning electron microscopy (SEM) image (80×magnification) of a cross-section of an extruded porous foam sheethaving an average pore size of 8 μm, a density of 0.989 g/cm³, and acell density of greater than 10⁶ cells per cm³.

[0015]FIG. 4 is a scanning electron microscopy (SEM) image (50×magnification) of the top surface of an extruded porous foam sheethaving an average pore size of 15 μm, an density of 0.989 g/cm³, a celldensity of greater than 10⁶ cells per cm³, and no surface macrotexture.

[0016]FIG. 5 is a plot of silicon dioxide removal rate versus the numberof silicon dioxide wafers polished using a microporous foam polishingpad of the invention.

[0017]FIG. 6 is a plot of silicon dioxide removal rate versus the numberof silicon dioxide wafers polished comparing a microporous foampolishing pad of the invention and a solid, non-porous polishing pad,wherein the polishing pads are grooved and buffed.

[0018]FIG. 7a is a scanning electron microscopy (SEM) image (20×magnification) of the top surface of a solid, non-porous polymer sheethaving a grooved macrotexture that is glazed and clogged with polishingdebris after polishing 20 silicon dioxide wafers, wherein the polishingpads are buffed and conditioned.

[0019]FIG. 7b is a scanning electron microscopy (SEM) image (20×magnification) of the top surface of an extruded porous foam sheethaving an average pore size of 15 μm, an density of 0.989 g/cm³, a celldensity of greater than 10⁶ cells per cm³, as well as a groovedmacrotexture that is free of polishing debris after polishing 20 silicondioxide wafers (buffed and conditioned).

[0020]FIG. 7c is a scanning electron microscopy (SEM) image (20×magnification) of the top surface of an extruded porous foam sheethaving an average pore size of 15 μm, an density of 0.989 g/cm³, a celldensity of greater than 10⁶ cells per cm³, as well as a groovedmacrotexture that is free of polishing debris after polishing 20 silicondioxide wafers (buffed, no conditioning).

[0021]FIGS. 8a, 8 b, and 8 c are Energy Dispersive X-ray (EDX) silicamapping images of a solid polishing pad (FIG. 8a), a microporous foampolishing pad of the invention (FIG. 8b), and a conventional closed cellpolishing pad (FIG. 8c) showing the extent of penetration of the silicaabrasive through the thickness of the polishing pad after polishing 20silicon dioxide blanket wafers.

[0022]FIG. 9 is a plot of time (s) versus the remaining step height (inÅ) for a 40% dense feature of a patterned silicon dioxide wafercomparing the use of a solid, non-porous polishing pad, a microporousfoam polishing pad of the invention, and a conventional microporousclosed cell polishing pad.

[0023] Figure 10 is a plot of time (s) versus the remaining step height(in Å) for a 70% dense feature of a patterned silicon dioxide wafercomparing the use of a solid, non-porous polishing pad, a microporousfoam polishing pad of the invention, and a conventional microporousclosed cell polishing pad.

[0024]FIG. 11 is an SEM image of a solid thermoplastic polyurethanesheet at a magnification of 350×.

[0025]FIG. 12 is an SEM image of a solid thermoplastic polyurethanesheet at a magnification of 7500× that has been treated by pressurizedgas injection to produce a foam having an average cell size of 0μm.

[0026]FIG. 13 is an SEM image of a solid thermoplastic polyurethanesheet at a magnification of 20000× that has been treated by pressurizedgas injection to produce a foam having an average cell size of 0μm.

[0027]FIG. 14 is an SEM image of a solid thermoplastic polyurethanesheet at a magnification of 350× that has been treated by pressurizedgas injection to produce a foam having an average cell size of 4 μm.

[0028]FIG. 15 is an SEM image of a solid thermoplastic polyurethanesheet at a magnification of 10000× that has been treated by pressurizedgas injection to produce a foam having an average cell size of 4 μm.

DETAILED DESCRIPTION OF THE INVENTION

[0029] In a first embodiment, the invention is directed to a polishingpad for chemical-mechanical polishing comprising a porous foam with anaverage pore size of about 50 μm or less. Preferably, the porous foamhas an average pore size of about 40 μm or less, or even about 30 μm orless (e.g. about 20 μm or less). Typically, the porous foam has anaverage pore size of about 1 μm or more (e.g., about 3 μm or more, orabout 5 μm or more).

[0030] In a second, preferred embodiment, the porous foam has an averagepore size of about 1 μm to about 20 μm. Preferably, the porous foam hasan average pore size of about 1 μm to about 15 μm (e.g., about 1 μm toabout 10 μm).

[0031] The porous foam of the polishing pads described herein has ahighly uniform distribution of pore sizes (i.e., cell sizes). Typically,about 75% or more (e.g., about 80% or more, or about 85% or more) of thepores (e.g., cells) in the porous foam have a pore size distribution ofabout ±20 μm or less (e.g., about ±10 μm or less, about ±5 μm or less,or about ±2 μm or less). In other words, about 75% or more (e.g., about80% or more, or about 85% or more) of the pores in the porous foam havea pore size within about 20 μm or less (e.g., about ±10 μm or less,about ±5 μm or less, or about ±2 μm or less) of the average pore size.Preferably, about 90% or more (e.g., about 93% or more, about 95% ormore, or about 97% or more) of the pores (e.g., cells) in the porousfoam have a pore size distribution of about ±20 μm or less (e.g., about±10 μm or less, about ±5 μm or less, or about ±2 μm or less).

[0032] Typically, the porous foam comprises predominantly closed cells(i.e., pores); however, the porous foam can also comprise open cells.Preferably, the porous foam comprises at least about 5% or more (e.g.,at least about 10% or more) closed cells. More preferably, the porousfoam comprises at least about 20% or more (e.g., at least about 40% ormore, or at least about 60% or more) closed cells.

[0033] The porous foam typically has a density of about 0.5 g/cm³ orgreater (e.g., about 0.7 g/cm³ or greater, or even about 0.9 g/cm³ orgreater) and a void volume of about 25% or less (e.g., about 15% orless, or even about 5% or less). Typically the porous foam has a celldensity of about 10⁵ cells/cm³ or greater (e.g., about 10⁶ cells/cm³ orgreater). The cell density can be determined by analyzing across-sectional image (e.g., an SEM image) of a porous foam materialwith an image analysis software program such as Optimase® imagingsoftware and ImagePro® imaging software, both by Media Cybernetics, orClemex Visione® imaging software by Clemex Technologies.

[0034] The porous foam can comprise any suitable material, typically apolymer resin. The porous foam preferably comprises a polymer resinselected from the group consisting of thermoplastic elastomers,thermoplastic polyurethanes, polyolefins, polycarbonates,polyvinylalcohols, nylons, elastomeric rubbers, styrenic polymers,polyaromatics, fluoropolymers, polyimides, cross-linked polyurethanes,cross-linked polyolefins, polyethers, polyesters, polyacrylates,elastomeric polyethylenes, polytetrafluoroethylenes,polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes,polystyrenes, polymethylmethacrylates, copolymers and block copolymersthereof, and mixtures and blends thereof. Preferably, the polymer resinis thermoplastic polyurethane.

[0035] The polymer resin typically is a pre-formed polymer resin;however, the polymer resin also can be formed in situ according to anysuitable method, many of which are known in the art (see, for example,Szycher's Handbook of Polyurethanes CRC Press: New York, 1999, Chapter3). For example, thermoplastic polyurethane can be formed in situ byreaction of urethane prepolymers, such as isocyanate, di-isocyanate, andtri-isocyanate prepolymers, with a prepolymer containing an isocyanatereactive moiety. Suitable isocyanate reactive moieties include aminesand polyols.

[0036] The selection of the polymer resin will depend, in part, on therheology of the polymer resin. Rheology is the flow behavior of apolymer melt. For Newtonian fluids, the viscosity is a constant definedby the ratio between the shear stress (i.e., tangential stress, σ) andthe shear rate (i.e., velocity gradient, dγ/dt). However, fornon-Newtonian fluids, shear rate thickening (dilatent) or shear ratethinning (pseudo-plastic) may occur. In shear rate thinning cases, theviscosity decreases with increasing shear rate. It is this property thatallows a polymer resin to be used in melt fabrication (e.g., extrusion,injection molding) processes. In order to identify the critical regionof shear rate thinning, the rheology of the polymer resins must bedetermined. The rheology can be determined by a capillary technique inwhich the molten polymer resin is forced under a fixed pressure througha capillary of a particular length. By plotting the apparent shear rateversus viscosity at different temperatures, the relationship between theviscosity and temperature can be determined. The Rheology ProcessingIndex (RPI) is a parameter that identifies the critical range of thepolymer resin. The RPI is the ratio of the viscosity at a referencetemperature to the viscosity after a change in temperature equal to 20°C. for a fixed shear rate. When the polymer resin is thermoplasticpolyurethane, the RPI preferably is about 2 to about 10 (e.g., about 3to about 8) when measured at a shear rate of about 150 l/s and atemperature of about 205° C.

[0037] Another polymer viscosity measurement is the Melt Flow Index(MFI) which records the amount of molten polymer (in grams) that isextruded from a capillary at a given temperature and pressure over afixed amount of time. For example, when the polymer resin isthermoplastic polyurethane or polyurethane copolymer (e.g., apolycarbonate silicone-based copolymer, a polyurethane fluorine-basedcopolymers, or a polyurethane siloxane-segmented copolymer), the MFIpreferably is about 20 or less (e.g., about 15 or less) over 10 minutesat a temperature of 210° C. and a load of 2160 g. When the polymer resinis an elastomeric polyolefin or a polyolefin copolymer (e.g., acopolymer comprising an ethylene a-olefin such as elastomeric or normalethylene-propylene, ethlene-hexene, ethylene-octene, and the like, anelastomeric ethylene copolymer made from metallocene based catalysts, ora polypropylene-styrene copolymer), the MFI preferably is about 5 orless (e.g., about 4 or less) over 10 minutes at a temperature of 210° C.and a load of 2160 g. When the polymer resin is a nylon orpolycarbonate, the MFI preferably is about 8 or less (e.g., about 5 orless) over 10 minutes at a temperature of 210° C. and a load of 2160 g.

[0038] The rheology of the polymer resin can depend on the molecularweight, polydispersity index (PDI), the degree of long-chain branchingor cross-linking, glass transition temperature (T_(g)), and melttemperature (T_(m)) of the polymer resin. When the polymer resin isthermoplastic polyurethane or polyurethane copolymer (such as thecopolymers described above), the weight average molecular weight (M_(w))is typically about 50,000 g/mol to about 300,000 g/mol, preferably about70,000 g/mol to about 150,000 g/mol, with a PDI of about 1.1 to about 6,preferably about 2 to about 4. Typically, the thermoplastic polyurethanehas a glass transition temperature of about 20° C. to about 110° C. anda melt transition temperature of about 120° C. to about 250° C. When thepolymer resin is an elastomeric polyolefin or a polyolefin copolymer(such as the copolymers described above), the weight average molecularweight (M_(w)) typically is about 50,000 g/mol to about 400,000 g/mol,preferably about 70,000 g/mol to about 300,000 g/mol, with a PDI ofabout 1.1 to about 12, preferably about 2 to about 10. When the polymerresin is nylon or polycarbonate, the weight average molecular weight(M_(w)) typically is about 50,000 g/mol to about 150,000 g/mol,preferably about 70,000 g/mol to about I 00,000 g/mol, with a PDI ofabout 1. I to about 5, preferably about 2 to about 4.

[0039] The polymer resin selected for the porous foam preferably hascertain mechanical properties. For example, when the polymer resin is athermoplastic polyurethane, the Flexural Modulus (ASTM D790) preferablyis about 350 MPa (˜50,000 psi) to about 1000 MPa (˜150,000 psi), theaverage % compressibility is about 7 or less, the average % rebound isabout 35 or greater, and the Shore D hardness (ASTM D2240-95) is about40 to about 90 (e.g., about 50 to about 80).

[0040] In a third embodiment, the polishing pad comprises a porous foamcomprising a thermoplastic polyurethane, wherein the porous foam has anaverage pore size of about 50 μm or less (e.g., about 40 μm or less, orabout 25 μm or less) and wherein the thermoplastic polyurethane has aMelt Flow Index (MFI) of about 20 or less, an RPI of about 2 to about 10(e.g., about 3 to about 8), and a molecular weight (MW) of about 50,000g/mol to about 300,000 g/mol, with a PDI of about 1.1 to about 6 (e.g.,about 2 to about 4). Preferably, the thermoplastic polyurethane has aFlexural Modulus of about 350 MPa (˜50,000 psi) to about 1000 MPa(˜150,000 psi), an average % compressibility of about 7 or less, anaverage % rebound of about 35 or greater, and a Shore D hardness ofabout 40 to about 90 (e.g., about 50 to about 80). Such a polishing padcan have one or more physical characteristics (e.g., pore size andpolymer properties) described herein for the other embodiments of theinvention.

[0041] In a fourth embodiment, the polishing pad is a polyurethanepolishing pad comprising a polishing surface, which in the absence ofany externally produced surface texture, can polish a silicon dioxidewafer with a polishing rate of at least about 600 Å/min with a carrierdownforce pressure of about 0.028 MPa (4 psi), a slurry flow rate ofabout 100 ml/min, a platen rotation speed of about 60 rpm, and a carrierrotation speed of about 55 rpm to about 60 rpm. The polishing pad of thefourth embodiment does not contain abrasive particles suspended in thefoam and is used in conjunction with a polishing composition (i.e.,slurry) containing metal oxide particles, in particular, Semi-Sperse®D7300 polishing composition sold by Cabot Microelectronics Corporation.Typically, the polishing pad can polish a silicon dioxide wafer with apolishing rate of at least about 800 Å/min or even at least about 1000Å/min using the polishing parameters recited above. The polishing padhas a void volume of about 25% or less and comprises pores having anaverage pore size of about 50 μm or less (e.g., about 40 μm or less).The polishing pad also can polish silicon dioxide blanket wafers suchthat the silicon dioxide blanket wafers have low non-uniformity valuesof only about 2% to about 4%. Such a polishing pad can have one or morephysical characteristics (e.g., pore size and polymer properties)described herein for the other embodiments of the invention.

[0042] In a fifth embodiment, the polishing pad comprises a porous foamhaving a multi-modal distribution of pore sizes. The term “multi-modal”means that the porous foam has a pore size distribution comprising atleast 2 or more (e.g., about 3 or more, about 5 or more, or even about10 or more) pore size maxima. Typically the number of pore size maximais about 20 or less (e.g., about 15 or less). A pore size maxima isdefined as a peak in the pore size distribution whose area comprisesabout 5% or more by number of the total number of pores. Preferably, thepore size distribution is bimodal (i.e., has two pore size maxima).

[0043] The multi-modal pore size distribution can have pore size maximaat any suitable pore size values. For example, the multi-modal pore sizedistribution can have a first pore size maximum of about 50 μm or less(e.g., about 40 μm or less, about 30 μm or less, or about 20 μm or less)and a second pore size maximum of about 50 μm or more (e.g., about 70 μmor more, about 90 μm or more, or even about 120 μm or more). Themulti-modal pore size distribution alternatively can have a first poresize maximum of about 20 μm or less (e.g., about 10 μm or less, or about5 μm or less) and a second pore size maximum of about 20 μm or more(e.g., about 35 μm or more, about 50 μm or more, or even about 75 μm ormore).

[0044] The porous foam of the fifth embodiment can comprise any suitablepolymer resin, for example, the porous foam can comprise any of thepolymer resins described herein. Preferably, the porous foam comprises athermoplastic polyurethane. The polymer resin can have any of thephysical, mechanical, or chemical properties described herein withrespect to the other embodiments.

[0045] The porous foam of the polishing pads described herein optionallyfurther comprises a water absorbent polymer. The water absorbent polymerdesirably is selected from the group consisting of amorphous,crystalline, or cross-linked polyacrylamide, polyacrylic acid,polyvinylalcohol, salts thereof, and combinations thereof. Preferably,the water absorbent polymers are selected from the group consisting ofcross-linked polyacrylamide, cross-linked polyacrylic acid, cross-linkedpolyvinylalcohol, and mixtures thereof. Such cross-linked polymersdesirably are water-absorbent but will not melt or dissolve in commonorganic solvents. Rather, the water-absorbent polymers swell uponcontact with water (e.g., the liquid carrier of a polishingcomposition).

[0046] The porous foam of the polishing pads described herein withrespect to the first, second, third, and fifth embodiments optionallycontains particles that are incorporated into the body of the pad.Preferably, the particles are dispersed throughout the porous foam. Theparticles can be abrasive particles, polymer particles, compositeparticles (e.g., encapsulated particles), organic particles, inorganicparticles, clarifying particles, and mixtures thereof. 100471 Theabrasive particles can be of any suitable material, for example, theabrasive particles can comprise a metal oxide, such as a metal oxideselected from the group consisting of silica, alumina, ceria, zirconia,chromia, iron oxide, and combinations thereof, or a silicon carbide,boron nitride, diamond, garnet, or ceramic abrasive material. Theabrasive particles can be hybrids of metal oxides and ceramics orhybrids of inorganic and organic materials. The particles also can bepolymer particles many of which are described in U.S. Pat. No.5,314,512, such as polystyrene particles, polymethylmethacrylateparticles, liquid crystalline polymers (LCP, e.g., Vectra® polymers fromCiba Geigy), polyetheretherketones (PEEK's), particulate thermoplasticpolymers (e.g., particulate thermoplastic polyurethane), particulatecross-linked polymers (e.g., particulate cross-linked polyurethane orpolyepoxide), or a combination thereof. If the porous foam comprises apolymer resin, then the polymer particle desirably has a melting pointthat is higher than the melting point of the polymer resin of the porousfoam. The composite particles can be any suitable particle containing acore and an outer coating. For example, the composite particles cancontain a solid core (e.g., a metal oxide, metal, ceramic, or polymer)and a polymeric shell (e.g., polyurethane, nylon, or polyethylene). Theclarifying particles can be phyllosilicates, (e.g., micas such asfluorinated micas, and clays such as talc, kaolinite, montmorillonite,hectorite), glass fibers, glass beads, diamond particles, carbon fibers,and the like.

[0047] The porous foam of the polishing pads described herein optionallycontains soluble particles incorporated into the body of the pad.Preferably, the soluble particles are dispersed throughout the porousfoam. Such soluble particles partially or completely dissolve in theliquid carrier of the polishing composition during chemical-mechanicalpolishing. Typically, the soluble particles are water-soluble particles.For example, the soluble particles can be any suitable water-solubleparticles such as particles of materials selected from the groupconsisting of dextrins, cyclodextrins, mannitol, lactose,hydroxypropylcelluloses, methylcelluloses, starches, proteins, amorphousnon-cross-linked polyvinyl alcohol, amorphous non-cross-linked polyvinylpyrrolidone, polyacrylic acid, polyethylene oxide, water-solublephotosensitive resins, sulfonated polyisoprene, and sulfonatedpolyisoprene copolymer. The soluble particles also can be an inorganicwater-soluble particles of materials selected from the group consistingof potassium acetate, potassium nitrate, potassium carbonate, potassiumbicarbonate, potassium chloride, potassium bromide, potassium phosphate,magnesium nitrate, calcium carbonate, and sodium benzoate. When thesoluble particles dissolve, the polishing pad can be left with openpores corresponding to the size of the soluble particle.

[0048] The particles preferably are blended with the polymer resinbefore being formed into a foamed polishing substrate. The particlesthat are incorporated into the polishing pad can be of any suitabledimension (e.g., diameter, length, or width) or shape (e.g., spherical,oblong) and can be incorporated into the polishing pad in any suitableamount. For example, the particles can have a particle dimension (e.g.,diameter, length, or width) of about 1 nm or more and/or about 2 mm orless (e.g., about 0.5 μm to about 2 mm diameter). Preferably, theparticles have a dimension of about 10 nm or more and/or about 500 μm orless (e.g., about 100 nm to about 10 μm diameter). The particles alsocan be covalently bound to the polymer resin of the porous foam.

[0049] The porous foam of the polishing pads described herein optionallycontains solid catalysts that are incorporated into the body of the pad.Preferably, the solid catalysts are dispersed throughout the porousfoam. The catalyst can be metallic, non-metallic, or a combinationthereof. Preferably, the catalyst is chosen from metal compounds thathave multiple oxidation states, such as but not limited to metalcompounds comprising Ag, Co, Ce, Cr, Cu, Fe, Mo, Mn, Nb, Ni, Os, Pd, Ru,Sn, Ti, and V.

[0050] The porous foam of the polishing pads described herein optionallycontains chelating agents or oxidizing agents. Preferably, the chelatingagents and oxidizing agents are dispersed throughout the porous foam.The chelating agents can be any suitable chelating agents. For example,the chelating agents can be carboxylic acids, dicarboxylic acids,phosphonic acids, polymeric chelating agents, salts thereof, and thelike. The oxidizing agents can be oxidizing salts or oxidizing metalcomplexes including iron salts, aluminum salts, peroxides, chlorates,perchlorates, permanganates, persulfates, and the like.

[0051] The polishing pads described herein have a polishing surfacewhich optionally further comprises grooves, channels, and/orperforations which facilitate the lateral transport of polishingcompositions across the surface of the polishing pad. Such grooves,channels, or perforations can be in any suitable pattern and can haveany suitable depth and width. The polishing pad can have two or moredifferent groove patterns, for example a combination of large groovesand small grooves as described in U.S. Pat. No. 5,489,233. The groovescan be in the form of slanted grooves, concentric grooves, spiral orcircular grooves, XY crosshatch pattern, and can be continuous ornon-continuous in connectivity. Preferably, the polishing pad comprisesat least small grooves produced by standard pad conditioning methods.

[0052] The polishing pads described herein have a polishing surfacewhich optionally further comprises regions of different density,porosity, hardness, modulus, and/or compressibility. The differentregions can have any suitable shape or dimension. Typically, the regionsof contrasting density, porosity, hardness, and/or compressibility areformed on the polishing pad by an ex situ process (i.e., after thepolishing pad is formed).

[0053] The polishing pads described herein optionally further compriseone or more apertures, transparent regions, or translucent regions(e.g., windows as described in U.S. Pat. No. 5,893,796). The inclusionof such apertures or translucent regions is desirable when the polishingpad is to be used in conjunction with an in situ CMP process monitoringtechnique. The aperture can have any suitable shape and may be used incombination with drainage channels for minimizing or eliminating excesspolishing composition on the polishing surface. The translucent regionor window can be any suitable window, many of which are known in theart. For example, the translucent region can comprise a glass orpolymer-based plug that is inserted in an aperture of the polishing pador may comprise the same polymeric material used in the remainder of thepolishing pad.

[0054] The polishing pads of the invention can be produced using anysuitable technique, many of which are known in the art. For example, thepolishing pads can be produced by (a) a mucell process, (b) a phaseinversion process, (c) a spinodal or bimodal decomposition process, or(d) a pressurized gas injection process. Preferably, the polishing padsare produced using the mucell process or the pressurized gas injectionprocess.

[0055] The mucell process involves (a) combining a polymer resin with asupercritical gas to produce a single-phase solution and (b) forming apolishing pad substrate of the invention from the single-phase solution.The polymer resin can be any of the polymer resins described above. Thesupercritical gas is generated by subjecting a gas to an elevatedtemperature and pressure sufficient to create a supercritical state inwhich the gas behaves like a fluid (i.e., a supercritical fluid, SCF).The gas can be a hydrocarbon, chlorofluorocarbon,hydrochlorofluorocarbon (e.g., freon), nitrogen, carbon dioxide, carbonmonoxide, or a combination thereof Preferably, the gas is anon-flammable gas, for example a gas that does not contain C—H bonds.More preferably, the gas is nitrogen, carbon dioxide, or a combinationthereof. Most preferably, the gas comprises, or is, carbon dioxide. Thegas can be converted to the supercritical gas before or aftercombination with the polymer resin. Preferably, the gas is converted tothe supercritical gas before combination with the polymer resin.Typically, the gas is subjected to a temperature of about 100° C. toabout 300° C. and a pressure of about 5 MPa (˜800 psi) to about 40 MPa(˜6000 psi). When the gas is carbon dioxide, the temperature is about150° C. to about 250 ° C., and the pressure is about 7 MPa (˜1000 psi)to about 35 MPa (˜5000 psi) (e.g., about 19 MPa (˜2800 psi) to about 26MPa (˜3800 psi)).

[0056] The single-phase solution of the polymer resin and thesupercritical gas can be prepared in any suitable manner. For example,the supercritical gas can be blended with molten polymer resin in amachine barrel to form the single-phase solution. The single-phasesolution then can be injected into a mold, where the gas expands to forma pore structure with high uniformity of pore size within the moltenpolymer resin. The concentration of the supercritical gas in thesingle-phase solution typically is about 0.01% to about 5% (e.g., about0.1% to about 3%) of the total volume of the single-phase solution. Theconcentration of the supercritical fluid will determine the density ofthe porous foam and the pore size. As the concentration of thesupercritical gas is increased, the density of the resulting porous foamincreases and the average pore size decreases. The concentration of thesupercritical gas also can affect the ratio of open cells to closedcells in the resulting porous foam. These and additional processfeatures are described in further detail in U.S. Pat. No. 6,284,810.

[0057] The polishing pad is formed by creating a thermodynamicinstability in the single-phase solution sufficient to produce greaterthan about 10⁵ nucleation sites per cm³, of the solution. Thethermodynamic instability can result from, for example, a rapid changein temperature, a rapid drop in pressure, or a combination thereof.Typically, the thermodynamic instability is induced at the exit of themold or die which contains the single-phase solution. Nucleation sitesare the sites at which the dissolved molecules of the supercritical gasform clusters from which the cells in the porous foam grow. The numberof nucleation sites is determined by assuming that the number ofnucleation sites is approximately equal to the number of cells formed inthe polymer foam. The polishing pad can be formed from the single-phasesolution by any suitable technique. For example, the polishing pad canbe formed using a technique selected from the group consisting ofextrusion into a polymer sheet, co-extrusion of multilayer sheets,injection molding, compression molding, blow molding, blown film,multilayer blown film, cast film, thermoforming, and lamination.Preferably, the polishing pad is formed by extrusion into a polymersheet or by injection molding.

[0058] The phase inversion process involves the dispersion of extremelyfine particles of a polymer resin that have been heated above the T_(m)or T_(g) of the polymer in a highly agitated non-solvent (e.g., giveexamples here). The polymer resin can be any of the polymer resinsdescribed above. As the number of fine polymer resin particles added tothe non-solvent increases, the fine polymer resin particles connect toform initially as tendrils and ultimately as a three-dimensional polymernetwork. The non-solvent mixture is then cooled causing the non-solventto form into discrete droplets within the three-dimensional polymernetwork. The resulting material is a polymer foam having sub-micron poresizes.

[0059] The spinodal or binodal decomposition process involvescontrolling the temperature and/or volume fraction of a polymer-polymermixture, or a polymer-solvent mixture, so as to move the mixture from asingle-phase region into a two-phase region. Within the two-phaseregion, either spinodal decomposition or binodal decomposition of thepolymer mixture can occur. Decomposition refers to the process by whicha polymer mixture changes from a nonequilibrium phase to an equilibriumphase. In the spinodal region, the free energy of mixing curve isnegative such that phase separation of the polymers (i.e., formation ofa two-phase material), or phase separation of the polymer and thesolvent, is spontaneous in response to small fluctuations in the volumefraction. In the binodal region, the polymer mixture is stable withrespect to small fluctuations in volume fraction and thus requiresnucleation and growth to achieve a phase-separated material.Precipitation of the polymer mixture at a temperature and volumefraction within the two-phase region (i.e., the binodal or spinodalregion) results in the formation of a polymer material having twophases. If the polymer mixture is laden with a solvent or a gas, thebiphasic polymer material will contain sub-micron pores at the interfaceof the phase-separation. The polymers preferably comprise the polymerresins described above.

[0060] The pressurized gas injection process involves the use of hightemperatures and pressures to force a supercritical fluid gas into asolid polymer sheet comprising an amorphous polymer resin. The polymerresin can be any of the polymer resins described above. Solid extrudedsheets are placed at room temperature into a pressure vessel. Asupercritical gas (e.g., N₂ or CO₂) is added to the vessel, and thevessel is pressurized to a level sufficient to force an appropriateamount of the gas into the free volume of the polymer sheet. The amountof gas dissolved in the polymer is directly proportional to the appliedpressure according to Henry's law. Increasing the temperature of thepolymer sheet increases the rate of difflusion of the gas into thepolymer, but also decreases the amount of gas that can dissolve in thepolymer sheet. Once the gas has thoroughly saturated the polymer, thesheet is removed from the pressurized vessel. If desired, the polymersheet can be quickly heated to a softened or molten state if necessaryto promote cell nucleation and growth. U.S. Pat. Nos. 5,182,307 and5,684,055 describe these and additional features of the pressurized gasinjection process.

[0061] The polishing pads of the invention are particularly suited foruse in conjunction with a chemical-mechanical polishing (CMP) apparatus.Typically, the apparatus comprises a platen, which, when in use, is inmotion and has a velocity that results from orbital, linear, or circularmotion, a polishing pad of the invention in contact with the platen andmoving with the platen when in motion, and a carrier that holds asubstrate to be polished by contacting and moving relative to he surfaceof the polishing pad intended to contact a substrate to be polished. Thepolishing of the substrate takes place by the substrate being placed incontact with the polishing pad and then the polishing pad movingrelative to the substrate, typically with a polishing compositiontherebetween, so as to abrade at least a portion of the substrate topolish the substrate. The CMP apparatus can be any suitable CMPapparatus, many of which are known in the art. The polishing pad of theinvention also can be used with linear polishing tools.

[0062] The polishing pads described herein can be used alone oroptionally can be used as one layer of a multi-layer stacked polishingpad. For example, the polishing pads can be used in combination with asubpad. The subpad can be any suitable subpad. Suitable subpads includepolyurethane foam subpads (e.g., Porong foam subpads from RogersCorporation), impregnated felt subpads, microporous polyurethanesubpads, or sintered urethane subpads. The subpad typically is softerthan the polishing pad of the invention and therefore is morecompressible and has a lower Shore hardness value than the polishing padof the invention. For example, the subpad can have a Shore A hardness ofabout 35 to about 50. In some embodiments, the subpad is harder, is lesscompressible, and has a higher Shore hardness than the polishing pad.The subpad optionally comprises grooves, channels, hollow sections,windows, aperatures, and the like. When the polishing pads of theinvention are used in combination with a subpad, typically there is anintermediate backing layer such as a polyethyleneterephthalate film,coextensive with and inbetween the polishing pad and the subpad.Alternatively, the porous foam of the invention also can be used as asubpad in conjunction with a conventional polishing pad.

[0063] The polishing pads described herein are suitable for use inpolishing many types of substrates and substrate materials. For example,the polishing pads can be used to polish a variety of substratesincluding memory storage devices, semiconductor substrates, and glasssubstrates. Suitable substrates for polishing with the polishing padsinclude memory disks, rigid disks, magnetic heads, MEMS devices,semiconductor wafers, field emission displays, and other microelectronicsubstrates, especially substrates comprising insulating layers (e.g.,silicon dioxide, silicon nitride, or low dielectric materials) and/ormetal-containing layers (e.g., copper, tantalum, tungsten, aluminum,nickel, titanium, platinum, ruthenium, rhodium, iridium or other noblemetals).

[0064] The following examples further illustrate the invention but, ofcourse, should not be construed as in any way limiting its scope.

EXAMPLE 1

[0065] This example illustrates a method for producing microporous foamrods having a uniform pore size.

[0066] Thermoplastic polyurethane (TPU) foam rods (1A and 1B) wereproduced by an extrusion method. Each TPU foam rod was prepared usingTPU (TT 1072 Tecothane® polyurethane, Thermedics Polymer Products)having a weight average molecular weight of 90,000 g/mol to 110,000g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placed in anextruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L/D singlescrew extruder) at elevated temperature and pressure to form a polymermelt. Carbon dioxide gas was injected into the polymer melt (using aTrexel TR30-5000G delivery system equipped with P7 trim and 4 standardinjectors) under the elevated temperature and pressure resulting information of a supercritical fluid CO₂ that blended with the polymermelt to form a single-phase solution. The CO₂/polymer solution wasextruded through a converging die (0.15 cm (0.060 inch) diameter, 12.1°angle) to form a porous foam rod. The concentration of CO₂ was 1.51% and1.26% for rods 1A and 1B, respectively.

[0067] The temperatures for each zone of the extruder, the gate, die andmelt temperatures, die pressure, screw speed, and concentration of CO₂are summarized in Table 1. A scanning electron microscopy (SEM) imagefor Rod Sample 1B is shown in FIG. 1. TABLE 1 Rod Rod ExtrusionParameters 1A 1B Zone 1 Temperature (° C.)  210 (410° F.)  210 (410° F.)Zone 2 Temperature (° C.)  221 (430° F.)  221 (430° F.) Zone 3Temperature (° C.)  221 (430° F.)  218 (425° F.) Zone 4 Temperature (°C.)  216 (420° F.)  204 (400° F.) Zone 5 Temperature (° C.)  216 (420°F.)  204 (400° F.) Gate Temperature (° C.)  227 (440° F.)  218 (425° F.)Die Temperature (° C.)  227 (440° F.)  218 (425° F.) Melt Temperature (°C.)  219 (427° F.)  212 (414° F.) Die Pressure P1 (MPa) 25.7 (3730 psi)24.5 (3560 psi) Die Pressure P2 (MPa) 20.7 (3010 psi) 21.2 (3080 psi)Die Pressure P3 (MPa) 19.7 (2860 psi) 20.3 (2940 psi) Die Pressure P4(MPa) 19.8 (2880 psi) 20.3 (2940 psi) Screw Speed (rpm)   14   13 DriveAmp (amp)   63   64 SCF Type CO₂ CO₂ SCF set (kg/hr) 0.40 (0.87 lb/hr)0.32 (0.70 lb/hr) Output (kg/hr) 26.3 (57.8 lb/hr) 25.3 (55.7 lb/hr) SCFConcentration (%) 1.51 1.26

[0068] This example illustrates that microporous foam materials havinguniform cell sizes can be produced using supercritical fluid microcelltechnology.

EXAMPLE 2

[0069] This example illustrates a method for preparing polishing pads ofthe invention.

[0070] A series of thermoplastic polyurethane (TPU) foam sheets (2A, 2B,2C, and 2D) were produced by an extrusion method. Each TPU sheet wasprepared using TPU (TT 1072 Tecothane® polyurethane, Thermedics PolymerProducts) having a weight average molecular weight of 90,000 g/mol to110,000 g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placedin an extruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L/Dsingle screw) at elevated temperature and pressure to form a polymermelt. Carbon dioxide gas was injected into the polymer melt under theelevated temperature and pressure resulting in formation of asupercritical fluid CO₂ that blended with the polymer melt to form asingle-phase solution. The CO₂/polymer solution was extruded through aflat die (30.5 cm (12 inch) wide, 0.005- 0.0036 cm (0.002- 0.0014 inch)flex gap, 6° converging) to form a porous foam sheet. The concentrationof CO₂ was 0.50%, 0.80%, 1.70%, and 1.95% for sheets 2A, 2B, 2C, and 2D,respectively.

[0071] The temperatures for each zone of the extruder, the gate, die andmelt temperatures, die pressure, screw speed, concentration of C0₂, andsheet dimensions are summarized in Table 2. TABLE 2 Extrusion ParametersSheet 2A Sheet 2B Sheet 2C Sheet 2D Zone 1 Temperature (° C.)  210 (410°F.)  210 (410° F.)  210 (410° F.)  210 (410° F.) Zone 2 Temperature (°C.)  221 (430° F.)  221 (430° F.)  221 (430° F.)  221 (430° F.) Zone 3Temperature (° C.)  221 (430° F.)  221 (430° F.)  213 (415° F.)  213(415° F.) Zone 4 Temperature (° C.)  216 (420° F.)  216 (420° F.)  190(375° F.)  190 (375° F.) Zone 5 Temperature (° C.)  216 (420° F.)  216(420° F.)  190 (375° F.)  190 (375° F.) Gate Temperature (° C.)  216(420° F.)  216 (420° F.)  204 (400° F.)  204 (400° F.) Die Temperature(° C.)  213 (415° F.)  213 (415° F.)  196 (385° F.)  196 (385° F.) MeltTemperature (° C.)  213 (415° F.)  213 (415° F.)  210 (410° F.)  210(410° F.) Die Pressure (MPa) 14.7 (2130 psi) 14.2 (2060 psi) 14.5 (2100psi) 14.3 (2070 psi) Screw Speed (rpm) 13 13 13 13 SCF Type CO₂ CO₂ CO₂CO₂ SCF set (kg/hr)  0.136  0.218  0.454  0.513 (0.30 lb/hr) (0.48lb/hr) (1.00 lb/hr) (1.13 lb/hr) Output (kg/hr) 27.2 27.2 26.3 26.3(60.0 lb/hr) (60.0 lb/hr) (58.0 lb/hr) (58.0 lb/hr) SCF Concentration(%)  0.50  0.80  1.70  1.95 Sheet Width (cm) 26.7 (10.5 in) 26.7 (10.5in) 25.4 (10 in) 27.3 (10.75 in) Sheet Thickness (cm)  0.0635  0.0711 0.108  0.108 (25 mil) (28 mil) (42.5 mil) (42.5 mil) Cell size largelarge and small small small

[0072] Porous TPU foam sheets having good uniformity of cell size (±25μm) were produced using each series of the extrusion parameters shown inTable 2. Samples 2A and 2B had large average cell sizes (>100 μm).Sheets 2C and 2D had small average cell sizes (<100 μm).

[0073] This example demonstrates that porous foam sheets having smallcell sizes can be produced by the supercritical fluid method.

EXAMPLE 3

[0074] This example illustrates a method for preparing polishing pads ofthe invention.

[0075] A series of thermoplastic polyurethane (TPU) foam sheets (3A, 3B,3C, and 3D) were produced by an extrusion method. Each TPU sheet wasprepared using TPU (TT 1072 Tecothane® polyurethane, Thermedics PolymerProducts) having a weight average molecular weight of 90,000 g/mol to110,000 g/mol with a PDI of 2.2 to 3.3. In each case, the TPU was placedin an extruder (Labex II primary, 6.35 cm (2.5 inch) diameter 32/1 L/Dsingle screw) at elevated temperature and pressure to form a polymermelt. Carbon dioxide gas-was injected into the polymer melt under theelevated temperature and pressure resulting in formation of asupercritical fluid CO₂ that blended with the polymer melt to form asingle-phase solution. The CO₂/polymer solution was extruded through aflat die (30.5 cm (12 inch) wide, 0.005- 0.0036 cm (0.002- 0.0014 inch)flex gap, 60° converging) to form a porous foam sheet. The concentrationof CO₂ was 1.38%, 1.50%, 1.66%, and 2.05% for sheets 3A, 3B, 3C, and 3D,respectively.

[0076] The temperatures for each zone of the extruder, the gate, die,and melt temperatures; die pressures, screw speed, and concentration ofCO₂ are summarized in Table 3. The average cell size produced in theporous TPU foam sheets depends on the concentration of the CO₂ gas. Aplot of the CO₂ concentration in the single-phase solution versus thedensity of the resulting sheets is shown in FIG. 2. TABLE 3 ExtrusionParameters Sheet 3A Sheet 3B Sheet 3C Sheet 3D Zone 1 Temperature (° C.) 214 (418° F.)  210 (410° F.)  210 (410° F.)  210 (410° F.) Zone 2Temperature (° C.)  221 (430° F.)  221 (430° F.)  221 (430° F.)  221(430° F.) Zone 3 Temperature (° C.)  213 (415° F.)  213 (415° F.)  213(415° F.)  213 (415° F.) Zone 4 Temperature (° C.)  193 (380° F.)  190(375° F.)  190 (375° F.)  179 (355° F.) Zone 5 Temperature (° C.)  193(380° F.)  190 (375° F.)  190 (375° F.)  179 (355° F.) Gate Temperature(° C.)  199 (390° F.)  193 (380° F.)  193 (380° F.)  185 (365° F.) Die 1Temperature (° C.)  199 (390° F.)  196 (385° F.)  196 (385° F.)  190(375° F.) Die 2 Temperature (° C.)  216 (420° F.)  210 (410° F.)  210(410° F.)  199 (390° F.) Melt Temperature (° C.)  210 (410° F.)  204(400° F.)  202 (395° F.)  196 (385° F.) Die Pressure P1 (MPa) 17.1 18.120.9 24.4 (2480 psi) (2630 psi) (3030 psi) (3540 psi) Die Pressure P2(MPa) 15.9 15.9 16.7 20.2 (2300 psi) (2310 psi) (2420 psi) (2930 psi)Die Pressure P3 (MPa) 13.4 14.3 15.4 18.6 (1950 psi) (2070 psi) (2230psi) (2700 psi) Die Pressure P4 (MPa) 12.9 13.8 14.7 17.7 (1870 psi)(2000 psi) (2130 psi) (2570 psi) Screw Speed (rpm) 13 13 13 13 Drive Amp(amp) — — 69 67 SCF Type CO₂ CO₂ CO₂ CO₂ SCF set (kg/hr)  0.363  0.399 0.454  0.513 (0.80 lb/hr) (0.88 lb/hr) (1.00 lb/hr) (1.13 lb/hr) Output(kg/hr) 26.3 26.3 27.3 25.0 (58 lb/hr) (58 lb/hr) (60.2 lb/hr) (55.1lb/hr) SCF Concentration (%)  1.38  1.50  1.66  2.05 Foam Sheet Width(cm) 27.9 28.6 27.9 27.3 (11.00 in) (11.25 in) (11.00 in) (10.75 in)Foam Sheet Thickness  0.100  0.104  0.103  0.100 (cm) (0.0395 in)(0.0410 in) (0.0407 in) (0.0395 in) Density (g/ml)  0.781  0.816  0.899 0.989

[0077] Porous TPU foam sheets having good uniformity of cell size wereproduced using each series of the extrusion parameters shown in Table 3.Scanning electron microscopy (SEM) images of Sample 3D are shown inFIGS. 3 (cross-section) and 4 (top surface). The physical properties ofSample 3D were determined, and the data are summarized in Table 4.

[0078] The polishing pad density was determined in accordance with theASTM D795 test method. The Shore A hardness of the polishing pad wasdetermined in accordance with the ASTM 2240 test method. The peak stressof the polishing pad was determined in accordance with the ASTM D638test method. The % compressibility was determined at 0.031 MPa (4.5 psi)pressure using an Ames meter. The probe of the Ames tester was firstzeroed (without the sample), and then the sample thickness was measured(D1). A 5-pound weight (0.031 MPa) was placed on the probe and thesample thickness was measured after 1 minute (D2). The compressibilityis the ratio of the difference in thickness (D1−D2) to the initialsample thickness (D1). The % compressibility was also measured using anInstron technique at a pressure of 0.5 MPa (72 psi). The % rebound wasdetermined using a Shore Resiliometer (Shore Instrument & MFG). The %rebound was measured in the height of the travel of a metal slug as itrebounds off the specimen preformed at 0.031 MPa (4.5 psi). The %rebound is reported as an average over 5 measurements. The flexuralmodulus was determined in accordance with the ASTM D790 test method. Theair permeability was determined using a Genuine Gurley 4340 AutomaticDensometer.

[0079] The T_(g) was determined either by Dynamic Mechanical Analyzer(DMA) or by Thermomechanical Analysis (TMA). For DMA, a TA 2980 modelinstrument was used at an operating temperature of −25° C. to 130° C., afrequency of 3 Hz, and a heating rate of 2.5 ° C./min. The T_(g) wascalculated from the midpoint of the storage modulus versus temperatureplot. For TMA, the test was performed in accordance with the ASTM E831test method. The T_(m) was determined by Differential ScanningColorimetry (DSC). A TA 2920 model instrument was used at an operatingtemperature of −50° C. to 230° C. and a heating rate of 10° C./min. TheT_(m) value was calculated form the peak melting point of the exothermicwave. The Storage Modulus was determined by DMA at 25° C. The Taber Wearis the amount of the porous foam sheet that is removed in 1000 cycles ofpolishing. The average pore size and pore density of the porous foamsheets were determined using SEM micrographs at 5× and 100×magnification.

[0080] The average pore size and pore size distribution were measured bycounting closed cell pores in a given unit area and then averaging thepore diameters using the imaging software, Clemex Vision softwareavailable from Clemex Technologies. The size and percentages for thepores are reported with respect to both width and length reflecting thenon-spherical nature of the pores in the sample. The pore density wasdetermined by the following formula:${{Number}\quad {of}\quad {cells}\text{/}{cm}^{3}} = {\left( {\frac{\rho_{solid}}{\rho_{{pad}\quad {material}}} - 1} \right)*\left( \frac{6}{\pi \quad d^{3}} \right)}$

[0081] where ρ_(solid) is the density of the solid thermoplasticpolyurethane pads (without SCF gas) equal to 1.2 g/cm³, ρpad material isthe density of the microcellular thermoplastic polyurethane pads (withSCF gas), and d is the diameter of the cell (in cm, assumed to bespherical). TABLE 4 Physical Property Value Thickness 0.107 cm (0.042in) Density 0.989 g/cm³ Shore A Hardness 93.7 Peak Stress 20.1 MPa(2911.8 psi) Average Pore Size (w × l) 7.9 μm ± 12.1 μm × 13.2 μm ± 20.6μm % Pores with Size 0-10 μm (w, l) 78.4, 61.2 % Pores with Size 10-20μm (w, l) 92.7, 84.7 % Pores with Size 20-30 μm (w, l) 96.8, 91.3 %Pores with Size ± 20 μm Average (w, l) 96, 91 Number of Cells per cm³ 47× 10⁶ % Compressibility @ 0.031 MPa (4.5 psi)  3.99% % Rebound @ 0.031MPa (4.5 psi) 46.11% Flexural Modulus 538 MPa (78,000 psi) Roughness14.66 μm Air Permeability 225.77 s T_(g) (DMA) 44.29° C. T_(m) (DSC) 80°C.-205° C. Storage Modulus @ 25° C. (DMA) 1000 MPa Taber Wear 71.65mg/1000 cycles

[0082] The average pore size and pore size distribution of the porousfoam of Sample 3D were also determined after the sample was conditionedwith a silicon oxide block for 5 hours. The values for the average poresize and the percentage of pore having a dimension within ±20 mm of theaverage (7.7±9.3 x 13.2±15.5 (w x 1) and 98%/91% (w/l), respectively)were substantially the same as the values obtained prior to conditioningand abrasion. These results indicate that the pore size and pore sizedistribution was consistent through the cross-sectional area of theporous foam sheet.

[0083] This example demonstrates that microporous polishing pads havinga uniform pore size can be prepared using the method of the invention.

EXAMPLE 4

[0084] This example illustrates that microporous foam polishing pads ofthe invention have good polishing properties.

[0085] A microporous foam polyurethane polishing pad produced accordingto the method recited in Example 3 for Sample 3D, having a density of0.989 g/ml and a thickness of 0.107 cm (0.0423 in), was used tochemically-mechanically polish blanket silicon dioxide wafers. Thepolishing pad was used without any conditioning (i.e., formation ofmicrogrooves or microstructure), buffing, or external macrogrooves(i.e., macrotexture). The removal rate and within wafer non-uniformitywas determined for the polishing pad as a function of the number ofsilicon dioxide wafers that were polished. The removal rates weremeasured for four wafers in a row followed by polishing of four “dummy”silicon dioxide wafers, for which removal rates were not recorded. Aplot of removal rate versus the number of silicon dioxide waferspolished is shown in FIG. 5. The polishing parameters were carrierdownforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100ml/min, a platen speed of 60 rpm, a carrier speed of 55-60 rpm.

[0086] The data depicted in FIG. 5 shows that polishing pads comprisinga microporous foam having a uniform cell size distribution producesubstantial polishing removal rates of silicon dioxide blanket wafers,even in the absence of any conditioning, buffing, or groovemacrotexture. Moreover, the polishing pads produce very low within wafernon-uniformity.

EXAMPLE 5

[0087] This example illustrates that microporous foam polishing pads ofthe invention have good polishing properties.

[0088] Different polishing pads were used to polish silicon dioxideblanket wafers in the presence of the same polishing composition (i.e.,Semi-Sperse® D7300 polishing composition sold by CabotMicroelectronics). Polishing Pad 5A (control) was a solid, non-porouspolyurethane polishing pad having microgrooves and macrogrooves.Polishing Pad 5B (invention) was a microporous foam polyurethanepolishing pad having a uniform pore size of 20±10 μm or less, which wasproduced according to the method recited in Example 3 for Sample 3D, andhaving a density of 0.989 g/ml and a thickness of 0.107 cm (0.0423 in)that was buffed, conditioned (to form microgrooves), and grooved(macrogrooves). The removal rates and non-uniformity were determined foreach of the polishing pads as a function of the number of silicondioxide wafers that were polished. A plot of removal rate versus thenumber of silicon dioxide wafers polished for each of the Polishing Pads5A and 5B is shown in FIG. 6. The polishing parameters were carrierdownforce pressure of 0.028 MPa (4 psi), a slurry flow rate of 100ml/min, a platen speed of 60 rpm, a carrier speed of 55-60 rpm. ScanningElectron Microscopy (SEM) images of the top grooved surfaces of thesolid polishing pad and microporous foam polishing pad of the inventionare shown in FIG. 7a and FIGS. 7b-7 c, respectively.

[0089] The plot of FIG. 6 shows that microporous foam polishing padshaving a uniform cell size distribution have superior removal rates forsilicon dioxide blanket wafers compared to solid, non-porous polishingpads. Moreover, the microporous polishing pad of the invention had avery consistent removal rate and low non-uniformity over the course ofpolishing 20 wafers or more, indicating that the polishing pad did notbecome glazed over time. The SEM images in FIGS. 7a-c illustrate thatthe microporous foam polishing pads of the invention (FIGS. 7b and 7 c)are less prone to glazing during polishing as is observed withconventional polishing pads (FIG. 7a).

EXAMPLE 6

[0090] This example illustrates that the microporous foam polishing padsof the invention are permeable to and can transport the polishingcomposition during polishing.

[0091] A solid polyurethane polishing pad (Pad 6A, comparative), amicroporous foam polyurethane polishing pad (Pad 6B, invention), and aconventional closed cell polyurethane polishing pad (Pad 6C,comparative) were used in a chemical-mechanical polishing experimentusing aqueous fumed silica abrasive at a pH of about 11. After polishing20 silicon dioxide wafers, each of the polishing pads were studied by aSEM X-ray mapping technique, Energy Dispersive X-ray (EDX) Spectroscopy,to determine the extent of penetration of the silica-based polishingcomposition. The EDX images are shown in FIGS. 8a, 8 b, and 8 c for Pads6A, 6B, and 6C, respectively.

[0092] The extent of penetration of the silica abrasive was only about10 or 15% of the pad thickness for the solid polishing pad (Pad 6A) asshown in FIG. 8a. For the microporous foam polishing pad (Pad 6B), thesilica abrasive penetrated through at least about 40% of the padthickness. For the conventional closed-cell polishing pad (Pad 6C), thesilica abrasive penetrated through only about 20% to 25% of the padthickness.

[0093] This example demonstrates that the microporous foam polishingpads of the invention are capable of transporting polishing compositionabrasive particles well into the body of the polishing pad, whileconventional solid and closed-cell polishing pads do not transport thepolishing composition into the body of the polishing pad.

EXAMPLE 7

[0094] This example shows that the microporous foam polishing pads ofthe invention have superior polishing rates compared to conventionalclosed cell microporous polishing pads.

[0095] Similar patterned silicon dioxide wafers were polished with anaqueous fumed silica abrasive at a pH of 11 using different polishingpads (Polishing Pads 7A, 7B, and 7C). Polishing Pad 7A (comparative) wasa solid non-porous polyurethane polishing pad. Polishing Pad 7B(invention) was a microporous foam polyurethane polishing pad of theinvention. Polishing Pad 7C (comparative) was a conventional microporousclosed cell polyurethane polishing pad. Each of the polishing pads werebuffed, conditioned, and grooved. The planarization rates for a 40%density region having a step height of 8000 Å and a 70% density regionhaving a step height of 8000 Å were polished by each of the polishingpads, and the remaining step height of the feature was determined after30, 60, 90, 120, and 150 seconds. The results for the 40% dense featureand the 70% dense feature are plotted in FIGS. 9 and 10, respectively.

[0096] The results depicted in FIGS. 9 and 10 show that for a region of40% density, all of the polishing pads (Polishing Pads 7A-7C) have lessthan 1000 Å remaining step height after 60 seconds. However, for aregion of 70% density, only Polishing Pads 7A and 7B have less than 1000Å remaining step height after 90 seconds. Thus, the microporous foampolishing pad of the invention has a superior polishing rate compared tothe conventional microporous foam closed cell polishing pad.

EXAMPLE 8

[0097] This example illustrates a method for preparing polishing pads ofthe invention using a pressurized gas injection process.

[0098] Two samples of solid extruded TPU sheets were placed in apressurized vessel with 5 MPa CO₂ gas at room temperature for about 30hours. The solid TPU sheets each absorbed about 5 wt.% CO₂. The TPUsamples (Samples 8A and 8B) were then heated to 50° C. and 97.6° C.,respectively, at a saturation pressure of 5 MPa to produce a sheet withan average cell size of 0.1 μm and 4 μm (99 cells counted, min 2 μm, max8 μm, standard deviation 1.5), respectively. The average cell sizes weredetermined using image analysis software. An SEM image of an untreatedsolid TPU sheet is shown in FIG. 11. SEM images of the foamed TPU sheets(Samples 8A and 8B, invention) are shown in FIGS. 12-15. FIGS. 12 and 13are at a magnification of 7500× and 20000×, respectively. FIGS. 14 and15 are at a magnification of 350× and 1000×, respectively.

[0099] This example demonstrates that the pressurized gas injectionprocess can be used to produce porous foam polishing pad materialshaving an average pore size less than 20 μm and a highly uniform poresize distribution.

[0100] All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

[0101] The use of the terms “a” and “an” and “the” and similar referentsin the context of describing the invention (especially in the context ofthe following claims) are to be construed to cover both the singular andthe plural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

[0102] Preferred embodiments of this invention are described herein,including the best mode known to the inventors for carrying out theinvention. Variations of those preferred embodiments may become apparentto those of ordinary skill in the art upon reading the foregoingdescription. The inventors expect skilled artisans to employ suchvariations as appropriate, and the inventors intend for the invention tobe practiced otherwise than as specifically described herein.Accordingly, this invention includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by theinvention unless otherwise indicated herein or otherwise clearlycontradicted by context.

What is claimed is:
 1. A polyurethane polishing pad forchemical-mechanical polishing which can polish a silicon dioxide waferat a rate of at least 600 Å/min with a carrier downforce pressure ofabout 0.028 MPa, a slurry flow rate of about 100 ml/min, a platenrotation speed of about 60 rpm, and a carrier rotation speed of about 55rpm to about 60 rpm, wherein the polishing pad does not contain abrasiveparticles and comprises no externally produced surface texture.
 2. Thepolishing pad of claim 1, wherein the polishing pad has a void volume ofabout 25% or less.
 3. The polishing pad of claim 2, wherein thepolishing pad has a void volume of about 5% or less.
 4. The polishingpad of claim 1, wherein the polishing pad comprises pores having anaverage pore size of about 50 μm or less.
 5. The polishing pad of claim4, wherein the polishing pad comprises pores having an average pore sizeof about 40 μm or less.
 6. The polishing pad of claim 1, wherein thepolyurethane has a Flexural Modulus of about 350 MPa to about 1000 MPa.7. The polishing pad of claim 1, wherein the polyurethane has a RheologyProcessing Index of about 2 to about 10 at a shear rate of about 150 l/sand a temperature of about 205° C.
 8. The polishing pad of claim 1,wherein the polyurethane has a glass transition temperature of about 20°C. to about 110° C. and a melt transition temperature of about 120 ° C.to about 250° C.
 9. The polishing pad of claim 1, wherein the polishingpad has an average % compressibility of about 7 or less, an average %rebound of about 35 or greater, and a Shore D hardness of about 40 toabout
 90. 10. The polishing pad of claim 1, wherein the polishing padfurther comprises a polymer resin selected from the group consisting ofthermoplastic elastomers, thermoplastic polyurethanes, polyolefins,polycarbonates, polyvinylalcohols, nylons, elastomeric rubbers, styrenicpolymers, polyaromatics, fluoropolymers, polyimides, cross-linkedpolyurethanes, cross-linked polyolefins, polyethers, polyesters,polyacrylates, elastomeric polyethylenes, polytetrafluoroethylenes,polyethyleneteraphthalates, polyimides, polyaramides, polyarylenes,polystyrenes, polymethylmethacrylates, copolymers and block copolymersthereof, and mixtures and blends thereof.
 11. The polishing pad of claim1, wherein the polishing pad further comprises a water-absorbentpolymer.
 12. The polishing pad of claim 11, wherein the water-absorbentpolymer is selected from the group consisting of cross-linkedpolyacrylamide, cross-linked polyacrylic acid, cross-linked polyvinylalcohol, and combinations thereof.
 13. The polishing pad of claim 1,wherein the polishing pad comprises closed cells.
 14. The polishing padof claim 1, wherein the polishing pad has a cell density of about 10⁵cells/cm³ or greater.
 15. The polishing pad of claim 1, wherein thepolishing pad has a bimodal pore size distribution.